Contrary to popular belief here, Apple did not attempt to patent all rectangles with rounded corners. They patented a particular set of design features. There is a reason they didn't sue B&N, Amazon, Asus, and so on, because the designs of those companies tablets were clearly outside of Apple's patent.
Similarly, Samsung could not file a design patent for the idea of a 7" tablet, or for all possible 7" tablets. They could patent the specific design features of their particular 7" tablet.

Apple's original iPhone patent included widgets.

Who said Apple attempted to patent "all rectangles with rounded corners?" Again, the filing of patents is not the issue. It's the way the holder wields those patents in costly and harmful litigation that's the real problem.

Sure, Samsung could file a design patent for smaller form tablets. It might eventually be denied, but at least they can file it.

As for the widgets, I suspect there is already a patent somewhere preceding Apple's which includes widgets in a mobile device. Otherwise it makes no sense that Apple has yet to introduce them to their iOS. Apple customers have been asking for them for years.